The manufacture of cast steel abrasive material, shot and grit, heretofore has been basically standard within the metal abrasives industry and cast steel shot and grit is sold subject to either the Society of Automotive Engineers (SAE) standards or the Steel Founders Society of America (SFSA) standards. The SFSA specification for cast steel abrasives carries the designation 20T-66 and is widely used. It sets forth technical requirements, particularly with regard to chemistry, microstructure, appearance and hardness. The process of manufacturing typically includes steps of: charging, melting, refining and pouring, with the pouring step being accomplished by directing the melted and refined molten steel through a stream of water under pressure and into a water quenching bath. The as-cast shot then, after being dried, is taken to a hardening furnace which is typically a continuously rotating retort having a series of gas burners controlling its temperature. After going into a quenching pit from the hardening furnace and again going through a drying operation, the hardened shot is subjected to a tempering operation to make tempered martensitic shot. The tempered martensitic shot ends up with a hardness on the Rc Scale of from 40 to 50. The hardened shot is crushed if grit is desired. Where softer shot was desired, a relatively long and expensive annealing operation was performed on as-cast shot. See, for example, U.S. Pat. No. 3,649,370 to Dorn et al.
The instant invention has been accomplished by consideration of the role which microstructure plays in the quality of steel shot and grit. Heretofore, tempered martensite has been accepted by the industry as the best microstructure and has been required by the above-mentioned standards.
The instant invention provides steel abrasive particles of improved breakdown life quality by selectively alternating the microstructure of the steel. It is known that in steel the micro-constituents pearlite, lower bainite, upper bainite and ferrite are softer and more ductile than either tempered or untempered martensite and have better abrasion resistance. Accordingly, the instant invention resulted from a refusal to accept martensite as the optimum of microstructure. The invention provides a microstructure which gives the steel particle better cleaning ability and prolonged useful life before the particle breaks down under a given set of use conditions.
It has been found that a particle of the more ductile constituents of upper bainite, lower bainite, pearlite or ferrite, when abraded repeatedly in the cleaning operation, undergoes slightly more deformation than a particle of tempered martensite. This is because the ductile particles tend to distribute impact and wear stresses more than the less ductile tempered martensite particles. It is believed that this behavior retards failure of the shot by reference since the actual stress levels are lower due to the distribution of wear forces over greater areas. Further, although wear stress levels of comparable ductile particles will be slightly lower than in tempered martensite, cleaning ability of the more ductile microstructures is comparable to or better than that of tempered martensite because the average particle size of the work mix of these softer microstructures will be larger than the average size of a comparable work mix of tempered martensite particles.
Moreover, the greater particle deflection results in slightly greater contact area between the shot cleaning media and the workpiece and this improves the area of effective cleaning of the particle. It has been found that shot and grit manufactured according to the instant invention and having the microstructure claimed provides an abrasive material which will clean as well as that subject to the current standards and will have a longer life. An important aspect of the invention is a new heat treating process for steel particles which permits production of upper bainite, lower bainite, pearlite and ferrite microstructures in the abrasive material to give it higher wear resistance and ductility.